For example, some diesel engines for automobiles have employed so-called exhaust gas recirculation (EGR) which extracts part of exhaust gas from an exhaust side to return the same to an intake side. The exhaust gas returned to the intake side suppresses combustion of fuel in the engine, thereby lowering the combustion temperature to reduce generation of NOx.
FIG. 1 shows an example of a diesel engine with a mechanism for recirculation of exhaust gas. In the diesel engine 1 shown, exhaust and intake passages 3 and 4 for communication of exhaust gas 2 are interconnected through an EGR pipe 5 having an EGR valve 6 through which the exhaust gas 2 is partly recirculated together with intake air 7 into each of cylinders for the diesel engine 1 so as to lower the combustion temperature in the cylinder and reduce the generation of NOx.
Each cylinder of the diesel engine 1 has a top (cylinder top 23) with a porous injector 8 for injection of fuel (diesel oil) into the cylinder, and a piston 9 having a top surface with a cavity 10 concave to provide a majority of a combustion chamber. To an inner periphery of the cavity 10, the fuel is radially injected via a tip of the injector 8 to self-ignite due to higher temperature in the cylinder during telophase of a compression stroke.
Particulars of the cavity 10 are as shown in FIG. 2. The cavity 10 is constituted by an inlet lip 11 which provides an outer edge of opening of the cavity 10, a combustion chamber wall 12 which extends down from the lip 11 in moderate S-shaped curve to protrude radially outwardly of the lip 11, an outer curved portion 13 extending from the wall 12 radially inwardly in moderate curved surface and a center cone 14 which is flatly conical and extends from an entire lower end circumference of the portion 13 to a piston center O.
Injection by the injector 8 in the diesel engine 1 shown in FIG. 1 is controlled by a fuel injection command 8a from a controller 15 which constitutes an engine-controlling computer (ECU: Electronic Control Unit) such that the command 8a is outputted to the injector 8 around compressive upper dead center so as to inject the fuel.
Inputted to the controller 15 are, for example, an accelerator stepped-in signal 16a from an accelerator sensor 16 (load sensor) for detection of an accelerator stepped-in degree as load to the diesel engine 1 and a revolution number signal 17a from a revolution number sensor 17 for detection of revolution number of the engine 1. Thus, operational conditions of the diesel engine 1 are monitored all the times so as to conduct various engine controls.
In FIG. 1, reference numeral 18 denotes a crank shaft; 19, an exhaust port; 20, an exhaust valve; 21, an intake port; 22, an intake valve; and 23 , the cylinder top. The intake and exhaust valves 22 and 20 are controlled to be opened by cams on engine-driven cam shafts through push rods and rocker arms (not shown) at appropriate timings depending upon strokes of the respective cylinders.
This kind of conventional combustion chamber structure for a direct injection diesel engine has been proposed, for example, in the following References 1 and 2.                [Reference 1] JP 6-212973A        [Reference 2] JP 7-150944A        